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A simplified procedure in radioisotope dilution method for the mercury inventory of electrolytic chlorine cells
International Journal of Applied Radiation and Isotopes, 1975, Vol. 26, pp. 671-675. Pergamon Pre~. Printed in Northern Ireland
A Simplified Procedure in Radioisotope Dilution Method for the Mercury Inventory of Electrolytic Chlorine Cells S. ENOMOTO, Y. KAWAKAMI, M. SENOO, T. IMAHASHI, N. TACHIKAWA and H. TOMINAGA Japan Atomic Energy Research Institute, Oaral-machi, Ibaraki-ken, Japan (P~doed 16 Janua~ 1975)
Concerning the radioisotope dilution method for determining the mercury amount in electr~ lyric ceils in the soda industry, a simple and safeprocedure with high accuracy was investigated. The procedure developed is based on the scrupulous supply by the Japan Atomic Energy Research Institute of the aliquots of radioactive mercury, i.e. t~I-Ig, prccisely weighed and the standard reference sample accurately diluted. The intended purpose is achieved with the s.d. in the determination less than 1~o INTRODUCTION THEmz ARE more than 50 chlorine plants in Japan producing several million tons of liquid chlorine annually; about 95 ~o of the amount is produced by means of the electrolysisof brine. Each electrolytic chlorine cell reqnires tons of metallic mercury for the cathode, flowing as a thin liquid sheet over the bed. Sodium forms the amalgam at the cathode, which then reacts with freshwater in the decomposer to release the sodium, after which the clean mercury is pumped back to the cell. The loss of the mercury in the electrolytic process has to be regularly examined, since the mercury is not only expensive but also possibly pollutes the environment. From the economic point of view, the total mass of mercury present in the cell and the consumption of mercury per ton of chlorine produced are both important. The conventional gravimetric method, involving the suspension of the operation and draining the mercury for weighing, needs considerable manpower and entails a loss of production; moreover, the results are not very accurate because of a significant amount trapped in the cell system. The radioisotope dilution method using radioactive mercury, t°¢Hg with a relatively 671
short half-life (65 hr), was proposed by COWLEY et al. ~t] It has advantages, for it does not disturb the operation while measuring the quantity of the mercury, and there is a negligible radioactivity after the measurement. Since 1967, therefore, this method has been practiced by some plants in this country.~z-~) However, it is not widely employed despite its advantages. This is because of the lack of a domestic supply of radioactive mercury and because of unfamiliarity with the handling of radioactive substances in the soda industry. Nevertheless, recent increases in the number of'cells per plant and also in the mercury amount per cell have presented difficulties in the continued use of the conventional method, and a simple radiometric method is highly desired. In view of this situation, the Japan Atomic Energy Research Institute (J.A.E.R.I.) has been giving technical guidance to many plants since 1970. During the years, the radiometric technique has been improved byJ.A.E.R.I, in terms of both simplicity and radiological safety. The procedure was thus established in a manual and offered to the Japan Soda Industry Association in May, 1973. At present, the method is successfully used in the soda industry. The method will be described in the present paper.
672
S. Enomoto, Y. Kawakami, M. Senoo, T. Imahashi, N. Tachikawa and H. Tominaga
PROCEDURE T h e procedure consists of the preparation and supply of a combination of aliquots of radioactive mercury to be introduced into the ceils and the standard references, and a mercury inventory of the electrolytic cells. T h e basic idea is to minimize the radiological work of the method in the field. F r o m this point of view, the process of diluting radioactive mercury to prepare the standard references is excluded from the field work; it is carried out at the Radioisotope Production Laboratory, J.A.E.R.I. Radioactive mercury, X97Hg, is produced as follows: (5) mercury oxide is irradiated by neutrons in a nuclear reactor. I t is then dissolved in nitric acid and converted to the metallic form by an isotopic-exchange reaction. After the moisture is removed, the respective precisely weighed mercury aliquots to be introduced into the ceils and the standard references quantitatively diluted from a part of the batch of radioactive mercury are prepared. T h e aliquots, enclosed in vials, are placed in lead containers with 5 m m thick walls and canned together, with soft tissue p a p e r and plastic as spacers. The n u m b e r of vials in a package corresponds to the n u m b e r of ceils, except for the standard references. O f the three standard references, two are sent to the plant, while the remaining one is kept at the laboratory in ease of trouble. T h e radioactive materials, with their specifications, are transported to various plants in the country by car or air. A block diagram of the procedure in the field is shown in Fig. 1. It consists of the addition of the radioactive mercury to cells, the inspection of the complete mixing in the mercury, and the sampling of the mixed mercury from the ceils in order to measure the radioactivity concentration. Mercury inventory using this method can be done in two or three days for a plant with several tens of cells. T o check the mixing of mercury in the ceils, four mercury samples are taken at different positions in a representative cell at suitable times after the radioactive-mercury addition. T h e standard deviation in the counts of the samples is calculated to compare with the theoretical error of the counting statistics. When the observed standard deviation of the four samples becomes comparable with the statistical error of
( Packoge from JAERI ) S ndard (r;a~ .... )
(tg?l.kj NO.I) ("7'HgNo.2) . . . . .
(l~HgNo.")
I
~ . .
~
• At electrolytic cells.
l
II
At
measuring room
....
....
FIG. 1. Block diagram showing a procedure proposed for the field work.
counting, the mixing is considered to be complete. W h e n this complete mixing is confirmed, mercury samples, two taken from each cell to eliminate operational mistakes, are used to measure the radioactivity concentration. T h e unknown quantity of mercury in a cell, M, can be obtained by the use of the following formula: M = mo{(ce/c ) R -- 1} where mo: quantity of radioactive mercury added to the cell; c: radiation count of the mercury taken from the cell after complete mixing; cs: radiation count of the standard reference mercury under the same measuring conditions; R: dilution factor of the standard reference mercury prepared from the batch. Ordinarily, the quantity of mercury in a cell, M , is of an order of I0 s-6 g, while that of the radioactive mercury added, m0, is about 10 ~ g; the ceR/c ratio is, then, more than 103. Consequently, the estimated standard deviation, a~,. in determining the quantity, M, is given approximately as:
Radioisotope dilution methodfor the mercu~ irwentoryoJ'electrolyti¢chlorine cells
/ /i
K Io
"
0
0
50 Mercury quontity
I00 (g)
150
FIO. 2. The relations between 7, X-ray count rate and radioactive mercury quantity in a glass test tube. By using a chemical balance, the errors of
g,,olmo and gR/R can be reduced to less than 0.1 ~o. T h e overall error thus depends largely on the measurement of the radioactivity concentrations; c and % T o realize a cr~[M error &less than 1 ~o in the presence of other errors, the statistical errors of the c and c, counts, must be held to less than 0.5 % by having more than 4 × 104 counts. T h e radioactivity measurement is performed with a scintillation counter connected to a scaler. T w o kinds of well-type N a I ( T I ) scintillators with a center hole and a through side hole, shown in Fig. 2, were examined for the measurement. I n measurement with a well-type scintillation counter, it is important to minimize the error caused by the irregularity in the samplemeasuring vessels. T h e glass test tubes commercially available are not too accurate in either diameter or wall thickness, so to avoid error only one test tube must be used for the measurement of both the sample and the standard reference. However, this results in noticeably troublesome work in the field. As an experiment, glass test tubes with equal dimensions were prepared by a special molding technique. As has been described in the literature, radioactive metallic mercury in an amount sufficient to achieve saturation in radiation counting is used for the measurement. This is because of the self-absorption at low energies o f 7(77 keV) and A u K X-rays (69 keV) from lg7Hg in the metallic mercury of a high density (13.6 g/ml).
673
Due to the inclusion of several percent of JWSHg with a 45-day half-life, the effective halflife of the radioactive mercury prepared is a little longer than that of l°THg. T h e radiochemical purity of the mercury depends on the neutron irradiation conditions in a nuclear reactor. T h e existence of a small amount of ~°SHg in the mercury presents a problem with the background in later measurements. I n 65-hr irradiation by means of thermal neutrons of about 1.2 X 10 is n[cm s sec flux density in a reactor (JRR-2), followed by 3-day processing in the laboratory, an effective halflife of about 85 hr was observed experimentally. T o avoid the need to correct for this decay, two samples taken from each cell are measured, before and after the standard reference, and the m e a n value of the counts in the two measurements is taken. T h e radiation counting time is practically limited in field work because of the short life of the radioactive mercury used. U n d e r those conditions, the time of background counting in minimizing the measurement error, tb, is given by t(N" nb[n) t/z, where N is the n u m b e r of samples measured per background counting, t is each sample counting time, and n and nb are the sample and the background count rate respectively.
R E S U L T S AND D I S C U S S I O N
Radioactivity measurement T h e results obtained for the saturation characteristics in the measurement of radioactive mercury are shown in Fig. 2. T h e saturation quantity of the mercury is about 100 g for both the scintillators. T h e detection efficiencies of the two are nearly the same, though the sidehole well-type is relatively small. T h e efficiency, then, depends on the surface area of a mercury sample. I t was shown b y experiments that the count rate increases with an increase in the tube diameter and a decrease in the wall thickness. About two hundred test tubes, half commercially available and half m a d e experimentally by ourselves were investigated in the radioactivity measurements, using the same mercury sample. T h e precisions in measurement by the centerhole weli-type scintillator were 1.43 and 0.62 ~o
S. Enomoto, Y. Kawakami, M. Senoo, T. Imahashi, N. Tarhihawa and H. Tominaga
674
respectively in a standard deviation for the two kinds of test tubes. With tubes of nearly equal outer diameters (within 0.05 ram, as measured by calipers), standard deviations of 0.58 and 0.12~o in the measurement were obtained for the center-hole and the side-hole well-type scintillators respectively. This shows that the bottoms of the test tubes, prepared manually, influence the measurement. T h e results obtained by the use of the side-hole well-type scintillator are superior to those obtained by means of the center-hole; however, the latter is usually used. After the above examination, 27 sets of 3 test tubes, 2 for the sample mercury and one for the standard reference, which coincided in counting within 0.1% in over l0 s counts were chosenrrom the total of 110 tubes made by the molding technique. These sets of tubes were delivered to the industry by J.A.E.R.I.
Complete mixing T h e time to attain the complete mixing is usually short as is shown in Fig. 3. During the mixing, buttered mercury floating on the surface of the flowing mercury and adhesive to the cell walls affects the mixing process. The measurement of the radioactivity in buttered mercury during the mixing process showed that the butter in the cell top-box was admixed with radioactive mercury in a short time, while the butter in the end-box took more than 30 hr to be t00 50 n=4,
~=1~4
so. In four mercury butter samples from different places, except for the cell end, with the cell cover removed, the radioactivity concentrations agreed within about 1 ~o at 20 hr after the radioactive-mercury addition. Because radioactive mercury can be mixed into the butter only slowly, it is better to take much time for the mixing. However, the amount of mercury butter in the total cathode mercury in the cell is normally less than 1 ~o, and the influence of the late mixing on the complete mixing can be practically neglected by having 24 hr, i.e. one full day, elapse before sampling. This period is also favorable for the work schedule. In the manual, therefore, a period o f 24 hr is allowed before sampling. It is also recommended to take samples from the cell topbox, avoiding the mercury butter.
Mercury inventory T h e precision of the radiometric method, i.e. the reproducibility of weighing the amount o f mercury in a cell, was also investigated. Table 1 compares the standard deviation derived from the count differences in every two samples and the statistical error in the radiation count, i.e. the square root of the count. It is shown that all the errors except the statistical error of the counting can be sufficiently suppressed by the present method. T h e accuracy of the present method was confirmed by comparing it with the conventional gravimetric one. T h e results obtained are shown in Table 2. T h e differences in value from the gravimetric method are about 4 ~ on the average. It should be noticed that the values were all larger than those of the conventional T,~T.~ 1. Comparison between the standard deviation derived from the measurement and the respective statistical error of radiation counting
t7
Plant
Number of cell
Standard deviation, measurement (~)
A B C D E
23 36 26 26 20
0.51 0.49 0.57 0.63 0.61
1 0.5
0=1
i
0.1 0.2
, ,[,t=ll
05
t Time
,
, , r,,q!l
2
5
,
10
20
r al
50
(hr)
Fio. 3. The features showing the mixing process of radioactive mercury into the cathode mercury in cell.
Statistical error of counting (~) 0.45 0.48 0.52 0.57 0-56
Radioisotope dilution methodfor the mercury inventory of electrolytic chlorinecells TABLE 2. The results obtained by the method proposed, comparing with that of conventional gravimetric method. Cell
Mercury quantity (kg) The difference Gravimetric Radlometric M e -- Mg method, Mg method, M r ~g" (%)
1 2 3 4 5
1001 940.5 966.5 968.4 956.7
1039 996-9 996.6 974.1 975.4
+3.80 +6-00 +3.11 +5.89 + 1.95
method, there was evidently some systematic deviation in this case. In order to demonstrate the validity of the present method, a secondary run in cells in which an appropriate amount of nonradioactive mercury had been added after the first run was carried out to determine the mercury amount. T h e results in T a b l e 3 indicate that the error in the standard deviation in the measurement of the mercury quantity in cells can be surely restrained within I ~o by the present method.
Radiological safety T h e radiological safety in the fieldwork is very m u c h improved by removing the process of preparing the standard references, as has been described above.
675
T h e 7-ray dose from the radioactive mercury (about 100 g) with 6 m C i contained in a vial is about 20 m r a d / h r at the mrface. T h e time necessary to add the mercury to the cells from the vials is about one minute per cell, so that the exposure of the operator's hands to radiation is not more than 10 m r e m for less than 30 cells. Measurement with finger film badges confirmed this figure. T h e concentration of radioactivity in the cell is normally less than 10-8 #Ci[g in the cathode mercury, and hardly a n y 7-ray dose on operators in the vicinity of the cells was observed in practice. CONCLUSION T h e amount of cathode mercury in the electrolytic chlorine cells can be determined safely and easily with an error (standard deviation) of less than 1 ~o by the present isotopedilution method using radioactive mercury 19~Hg of a 65-hr haft-llfe. Consequently, the work required can be considerably reduced, compared with the conventional radiometHc method; therefore it should be acceptable to the soda industry.
Acknowledgements--The authors wish to express their gratitude to Dr. K~Njx M o T o j ~ ofj.A.E.R.I., for his advice in the present work, and also to the colleagues at J.A.E.R.I. for their cooperation.
TABLE 3. The results obtained for the demonstration test, determining the additional mercury quantity
((M2--MI).--AM)
Mercury quantity (kg) Call 1 2 3 4 5 6
1st run Mx
added AM
2nd run M~
M2 -- M1
M2 (~o)
978 1022 1959 2417 2611 3770
I00 50 207 241 690 35
1080 1076 2153 2621 3309 3827
102 54 194 204 698 57
+0-19 +0.37 --0.60 --1.41 +0.24 +0.57 Std. dev.: 0"70(~o)
3. HATAYAMA T. and YOSHIDAd. ,,,Codaand CMorine REFERENCES 1. COWLey W. E., LoTr B. and BRow~ S. Chem/ca/ (in Japanese) 19 (I), I (1968). Engineer, No. 204, 345 (1966). 4. MORISHrrAH. Nud. Engin. (in Japanese) 17 (11), 2. N ~ s H ~ K., OSHn~ T. and I-ImxY~tA T. Proc. 76 (1971). 8th Japan Conf. on Radioisotopes (in Japanese), p. 285 5. YAMABAYASHIN., ONOUA K., MOTOISH! S. et al. (1967). Report JAERI-M 5320 (1973).
A Simplified Procedure in Radioisotope Dilution Method for the Mercury Inventory of Electrolytic Chlorine Cells S. ENOMOTO, Y. KAWAKAMI, M. SENOO, T. IMAHASHI, N. TACHIKAWA and H. TOMINAGA Japan Atomic Energy Research Institute, Oaral-machi, Ibaraki-ken, Japan (P~doed 16 Janua~ 1975)
Concerning the radioisotope dilution method for determining the mercury amount in electr~ lyric ceils in the soda industry, a simple and safeprocedure with high accuracy was investigated. The procedure developed is based on the scrupulous supply by the Japan Atomic Energy Research Institute of the aliquots of radioactive mercury, i.e. t~I-Ig, prccisely weighed and the standard reference sample accurately diluted. The intended purpose is achieved with the s.d. in the determination less than 1~o INTRODUCTION THEmz ARE more than 50 chlorine plants in Japan producing several million tons of liquid chlorine annually; about 95 ~o of the amount is produced by means of the electrolysisof brine. Each electrolytic chlorine cell reqnires tons of metallic mercury for the cathode, flowing as a thin liquid sheet over the bed. Sodium forms the amalgam at the cathode, which then reacts with freshwater in the decomposer to release the sodium, after which the clean mercury is pumped back to the cell. The loss of the mercury in the electrolytic process has to be regularly examined, since the mercury is not only expensive but also possibly pollutes the environment. From the economic point of view, the total mass of mercury present in the cell and the consumption of mercury per ton of chlorine produced are both important. The conventional gravimetric method, involving the suspension of the operation and draining the mercury for weighing, needs considerable manpower and entails a loss of production; moreover, the results are not very accurate because of a significant amount trapped in the cell system. The radioisotope dilution method using radioactive mercury, t°¢Hg with a relatively 671
short half-life (65 hr), was proposed by COWLEY et al. ~t] It has advantages, for it does not disturb the operation while measuring the quantity of the mercury, and there is a negligible radioactivity after the measurement. Since 1967, therefore, this method has been practiced by some plants in this country.~z-~) However, it is not widely employed despite its advantages. This is because of the lack of a domestic supply of radioactive mercury and because of unfamiliarity with the handling of radioactive substances in the soda industry. Nevertheless, recent increases in the number of'cells per plant and also in the mercury amount per cell have presented difficulties in the continued use of the conventional method, and a simple radiometric method is highly desired. In view of this situation, the Japan Atomic Energy Research Institute (J.A.E.R.I.) has been giving technical guidance to many plants since 1970. During the years, the radiometric technique has been improved byJ.A.E.R.I, in terms of both simplicity and radiological safety. The procedure was thus established in a manual and offered to the Japan Soda Industry Association in May, 1973. At present, the method is successfully used in the soda industry. The method will be described in the present paper.
672
S. Enomoto, Y. Kawakami, M. Senoo, T. Imahashi, N. Tachikawa and H. Tominaga
PROCEDURE T h e procedure consists of the preparation and supply of a combination of aliquots of radioactive mercury to be introduced into the ceils and the standard references, and a mercury inventory of the electrolytic cells. T h e basic idea is to minimize the radiological work of the method in the field. F r o m this point of view, the process of diluting radioactive mercury to prepare the standard references is excluded from the field work; it is carried out at the Radioisotope Production Laboratory, J.A.E.R.I. Radioactive mercury, X97Hg, is produced as follows: (5) mercury oxide is irradiated by neutrons in a nuclear reactor. I t is then dissolved in nitric acid and converted to the metallic form by an isotopic-exchange reaction. After the moisture is removed, the respective precisely weighed mercury aliquots to be introduced into the ceils and the standard references quantitatively diluted from a part of the batch of radioactive mercury are prepared. T h e aliquots, enclosed in vials, are placed in lead containers with 5 m m thick walls and canned together, with soft tissue p a p e r and plastic as spacers. The n u m b e r of vials in a package corresponds to the n u m b e r of ceils, except for the standard references. O f the three standard references, two are sent to the plant, while the remaining one is kept at the laboratory in ease of trouble. T h e radioactive materials, with their specifications, are transported to various plants in the country by car or air. A block diagram of the procedure in the field is shown in Fig. 1. It consists of the addition of the radioactive mercury to cells, the inspection of the complete mixing in the mercury, and the sampling of the mixed mercury from the ceils in order to measure the radioactivity concentration. Mercury inventory using this method can be done in two or three days for a plant with several tens of cells. T o check the mixing of mercury in the ceils, four mercury samples are taken at different positions in a representative cell at suitable times after the radioactive-mercury addition. T h e standard deviation in the counts of the samples is calculated to compare with the theoretical error of the counting statistics. When the observed standard deviation of the four samples becomes comparable with the statistical error of
( Packoge from JAERI ) S ndard (r;a~ .... )
(tg?l.kj NO.I) ("7'HgNo.2) . . . . .
(l~HgNo.")
I
~ . .
~
• At electrolytic cells.
l
II
At
measuring room
....
....
FIG. 1. Block diagram showing a procedure proposed for the field work.
counting, the mixing is considered to be complete. W h e n this complete mixing is confirmed, mercury samples, two taken from each cell to eliminate operational mistakes, are used to measure the radioactivity concentration. T h e unknown quantity of mercury in a cell, M, can be obtained by the use of the following formula: M = mo{(ce/c ) R -- 1} where mo: quantity of radioactive mercury added to the cell; c: radiation count of the mercury taken from the cell after complete mixing; cs: radiation count of the standard reference mercury under the same measuring conditions; R: dilution factor of the standard reference mercury prepared from the batch. Ordinarily, the quantity of mercury in a cell, M , is of an order of I0 s-6 g, while that of the radioactive mercury added, m0, is about 10 ~ g; the ceR/c ratio is, then, more than 103. Consequently, the estimated standard deviation, a~,. in determining the quantity, M, is given approximately as:
Radioisotope dilution methodfor the mercu~ irwentoryoJ'electrolyti¢chlorine cells
/ /i
K Io
"
0
0
50 Mercury quontity
I00 (g)
150
FIO. 2. The relations between 7, X-ray count rate and radioactive mercury quantity in a glass test tube. By using a chemical balance, the errors of
g,,olmo and gR/R can be reduced to less than 0.1 ~o. T h e overall error thus depends largely on the measurement of the radioactivity concentrations; c and % T o realize a cr~[M error &less than 1 ~o in the presence of other errors, the statistical errors of the c and c, counts, must be held to less than 0.5 % by having more than 4 × 104 counts. T h e radioactivity measurement is performed with a scintillation counter connected to a scaler. T w o kinds of well-type N a I ( T I ) scintillators with a center hole and a through side hole, shown in Fig. 2, were examined for the measurement. I n measurement with a well-type scintillation counter, it is important to minimize the error caused by the irregularity in the samplemeasuring vessels. T h e glass test tubes commercially available are not too accurate in either diameter or wall thickness, so to avoid error only one test tube must be used for the measurement of both the sample and the standard reference. However, this results in noticeably troublesome work in the field. As an experiment, glass test tubes with equal dimensions were prepared by a special molding technique. As has been described in the literature, radioactive metallic mercury in an amount sufficient to achieve saturation in radiation counting is used for the measurement. This is because of the self-absorption at low energies o f 7(77 keV) and A u K X-rays (69 keV) from lg7Hg in the metallic mercury of a high density (13.6 g/ml).
673
Due to the inclusion of several percent of JWSHg with a 45-day half-life, the effective halflife of the radioactive mercury prepared is a little longer than that of l°THg. T h e radiochemical purity of the mercury depends on the neutron irradiation conditions in a nuclear reactor. T h e existence of a small amount of ~°SHg in the mercury presents a problem with the background in later measurements. I n 65-hr irradiation by means of thermal neutrons of about 1.2 X 10 is n[cm s sec flux density in a reactor (JRR-2), followed by 3-day processing in the laboratory, an effective halflife of about 85 hr was observed experimentally. T o avoid the need to correct for this decay, two samples taken from each cell are measured, before and after the standard reference, and the m e a n value of the counts in the two measurements is taken. T h e radiation counting time is practically limited in field work because of the short life of the radioactive mercury used. U n d e r those conditions, the time of background counting in minimizing the measurement error, tb, is given by t(N" nb[n) t/z, where N is the n u m b e r of samples measured per background counting, t is each sample counting time, and n and nb are the sample and the background count rate respectively.
R E S U L T S AND D I S C U S S I O N
Radioactivity measurement T h e results obtained for the saturation characteristics in the measurement of radioactive mercury are shown in Fig. 2. T h e saturation quantity of the mercury is about 100 g for both the scintillators. T h e detection efficiencies of the two are nearly the same, though the sidehole well-type is relatively small. T h e efficiency, then, depends on the surface area of a mercury sample. I t was shown b y experiments that the count rate increases with an increase in the tube diameter and a decrease in the wall thickness. About two hundred test tubes, half commercially available and half m a d e experimentally by ourselves were investigated in the radioactivity measurements, using the same mercury sample. T h e precisions in measurement by the centerhole weli-type scintillator were 1.43 and 0.62 ~o
S. Enomoto, Y. Kawakami, M. Senoo, T. Imahashi, N. Tarhihawa and H. Tominaga
674
respectively in a standard deviation for the two kinds of test tubes. With tubes of nearly equal outer diameters (within 0.05 ram, as measured by calipers), standard deviations of 0.58 and 0.12~o in the measurement were obtained for the center-hole and the side-hole well-type scintillators respectively. This shows that the bottoms of the test tubes, prepared manually, influence the measurement. T h e results obtained by the use of the side-hole well-type scintillator are superior to those obtained by means of the center-hole; however, the latter is usually used. After the above examination, 27 sets of 3 test tubes, 2 for the sample mercury and one for the standard reference, which coincided in counting within 0.1% in over l0 s counts were chosenrrom the total of 110 tubes made by the molding technique. These sets of tubes were delivered to the industry by J.A.E.R.I.
Complete mixing T h e time to attain the complete mixing is usually short as is shown in Fig. 3. During the mixing, buttered mercury floating on the surface of the flowing mercury and adhesive to the cell walls affects the mixing process. The measurement of the radioactivity in buttered mercury during the mixing process showed that the butter in the cell top-box was admixed with radioactive mercury in a short time, while the butter in the end-box took more than 30 hr to be t00 50 n=4,
~=1~4
so. In four mercury butter samples from different places, except for the cell end, with the cell cover removed, the radioactivity concentrations agreed within about 1 ~o at 20 hr after the radioactive-mercury addition. Because radioactive mercury can be mixed into the butter only slowly, it is better to take much time for the mixing. However, the amount of mercury butter in the total cathode mercury in the cell is normally less than 1 ~o, and the influence of the late mixing on the complete mixing can be practically neglected by having 24 hr, i.e. one full day, elapse before sampling. This period is also favorable for the work schedule. In the manual, therefore, a period o f 24 hr is allowed before sampling. It is also recommended to take samples from the cell topbox, avoiding the mercury butter.
Mercury inventory T h e precision of the radiometric method, i.e. the reproducibility of weighing the amount o f mercury in a cell, was also investigated. Table 1 compares the standard deviation derived from the count differences in every two samples and the statistical error in the radiation count, i.e. the square root of the count. It is shown that all the errors except the statistical error of the counting can be sufficiently suppressed by the present method. T h e accuracy of the present method was confirmed by comparing it with the conventional gravimetric one. T h e results obtained are shown in Table 2. T h e differences in value from the gravimetric method are about 4 ~ on the average. It should be noticed that the values were all larger than those of the conventional T,~T.~ 1. Comparison between the standard deviation derived from the measurement and the respective statistical error of radiation counting
t7
Plant
Number of cell
Standard deviation, measurement (~)
A B C D E
23 36 26 26 20
0.51 0.49 0.57 0.63 0.61
1 0.5
0=1
i
0.1 0.2
, ,[,t=ll
05
t Time
,
, , r,,q!l
2
5
,
10
20
r al
50
(hr)
Fio. 3. The features showing the mixing process of radioactive mercury into the cathode mercury in cell.
Statistical error of counting (~) 0.45 0.48 0.52 0.57 0-56
Radioisotope dilution methodfor the mercury inventory of electrolytic chlorinecells TABLE 2. The results obtained by the method proposed, comparing with that of conventional gravimetric method. Cell
Mercury quantity (kg) The difference Gravimetric Radlometric M e -- Mg method, Mg method, M r ~g" (%)
1 2 3 4 5
1001 940.5 966.5 968.4 956.7
1039 996-9 996.6 974.1 975.4
+3.80 +6-00 +3.11 +5.89 + 1.95
method, there was evidently some systematic deviation in this case. In order to demonstrate the validity of the present method, a secondary run in cells in which an appropriate amount of nonradioactive mercury had been added after the first run was carried out to determine the mercury amount. T h e results in T a b l e 3 indicate that the error in the standard deviation in the measurement of the mercury quantity in cells can be surely restrained within I ~o by the present method.
Radiological safety T h e radiological safety in the fieldwork is very m u c h improved by removing the process of preparing the standard references, as has been described above.
675
T h e 7-ray dose from the radioactive mercury (about 100 g) with 6 m C i contained in a vial is about 20 m r a d / h r at the mrface. T h e time necessary to add the mercury to the cells from the vials is about one minute per cell, so that the exposure of the operator's hands to radiation is not more than 10 m r e m for less than 30 cells. Measurement with finger film badges confirmed this figure. T h e concentration of radioactivity in the cell is normally less than 10-8 #Ci[g in the cathode mercury, and hardly a n y 7-ray dose on operators in the vicinity of the cells was observed in practice. CONCLUSION T h e amount of cathode mercury in the electrolytic chlorine cells can be determined safely and easily with an error (standard deviation) of less than 1 ~o by the present isotopedilution method using radioactive mercury 19~Hg of a 65-hr haft-llfe. Consequently, the work required can be considerably reduced, compared with the conventional radiometHc method; therefore it should be acceptable to the soda industry.
Acknowledgements--The authors wish to express their gratitude to Dr. K~Njx M o T o j ~ ofj.A.E.R.I., for his advice in the present work, and also to the colleagues at J.A.E.R.I. for their cooperation.
TABLE 3. The results obtained for the demonstration test, determining the additional mercury quantity
((M2--MI).--AM)
Mercury quantity (kg) Call 1 2 3 4 5 6
1st run Mx
added AM
2nd run M~
M2 -- M1
M2 (~o)
978 1022 1959 2417 2611 3770
I00 50 207 241 690 35
1080 1076 2153 2621 3309 3827
102 54 194 204 698 57
+0-19 +0.37 --0.60 --1.41 +0.24 +0.57 Std. dev.: 0"70(~o)
3. HATAYAMA T. and YOSHIDAd. ,,,Codaand CMorine REFERENCES 1. COWLey W. E., LoTr B. and BRow~ S. Chem/ca/ (in Japanese) 19 (I), I (1968). Engineer, No. 204, 345 (1966). 4. MORISHrrAH. Nud. Engin. (in Japanese) 17 (11), 2. N ~ s H ~ K., OSHn~ T. and I-ImxY~tA T. Proc. 76 (1971). 8th Japan Conf. on Radioisotopes (in Japanese), p. 285 5. YAMABAYASHIN., ONOUA K., MOTOISH! S. et al. (1967). Report JAERI-M 5320 (1973).